The present invention relates to linear accelerators of the drift tube type known as linac. More particularly the present invention relates to a linac structure which achieves relatively high proton velocities in a short physical distance by means of alternating phase focusing. By employing alternating phase focusing structures in a linac, it is possible to accelerate protons and heavy ions employing higher frequencies, and from lower energies than has been possible heretofore with conventional magnetic quadrupole focus drift tube linac structures. While the acceleration rate is less than in the conventional drift tube structure due to the employment of the accelerating potential for focusing, the overall size and cost of the structure is decreased considerably. Exemplarily, a 400 MHz frequency and a relatively low injection energy of 250 keV may be employed. By arranging the drift tube lengths and gap positions, the particles can be made to experience acceleration and a succession of focusing and defocusing forces which result in a satisfactory containment of the beam without dependence on magnetic focusing fields. Therefore, the drift tubes can be smaller and shorter, allowing the structure to be extended to higher frequencies and lower energies than previously possible.
In the alternating phase focused linac structure, the gap-to-gap distances between drift tubes are arranged so that the particles are exposed to the rf fields at some periodic sequence of phase values alternating from side to side of the peak accelerating phase. This periodic modulation in the gap-to-gap distance implies a period modulation in the drift tube and geometries. These geometries must satisfy a number of constraints related to the dynamics of the particles which are to be accelerated, and the resonant frequency of the resulting structure.
In conventional linacs, the drift tubes include focusing magnets. In the early stages, drift tubes are small due to the relatively low particle velocity and low rf frequency. Therefore, the minimum practicable size drift tube that can be constructed limits the lower limit of required energy of injected ions or protons, and the highest rf frequency which can be employed. Conventional linacs operate at a maximum frequency of approximately 200 Mc, and require particle injection energies of about 750 keV, in order to make the first drift tubes large enough to be manageable.
In contrast to the linacs previously known to the art, the linac of the present invention does not require focusing magnets. They can, therefore, be considerably smaller and simpler than conventional drift tubes. Much lower injected energies are required, exemplarily 250 keV. Higher rf frequencies may be employed, exemplarily 400 Mc. As a result of the higher rf frequency, the tank diameter is halved, from 80 cm to 40 cm.
It will be apparent to one skilled in the art that a structure such as described above results in a smaller, simpler, less expensive linac. Lower injection energies enable much smaller injection systems, a major cost reduction. The higher frequency enables a much smaller diameter, shorter, tank. Further, the drift tubes themselves are much simpler to fabricate and mount. The lower cost, smaller overall structure resulting enables uses of linacs which had previously been prohibitively expensive, such as medical applications.